Method and device for desalinating water while overcoming decreases in pressure

ABSTRACT

The invention concerns a method of and an apparatus for continuously desalinating water by reverse osmosis, in particular desalinating sea water, wherein salt water is introduced under a first pressure by means of a delivery pump into a pressure compensating device, salt water is continuously introduced from the pressure compensating device at a second increased pressure into a membrane module and separated therein by means of a membrane into desalinated water and concentrated salt water, and the concentrated salt water discharged from the membrane module is continuously introduced under approximately the second pressure into the pressure compensating device and used there for acting with approximately the second pressure on the salt water introduced into the pressure compensating device and for introducing the salt water into the membrane module. In order to avoid disturbances in operation and possibly damage to the membrane because of a reduced flow over the membrane surface, it is provided in accordance with the invention that a continuous flow of the salt water introduced into the membrane module is maintained over the surface of the membrane by means of salt water discharged from a reservoir.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention concerns a method and a corresponding apparatus forcontinuously desalinating water by reverse osmosis, in particular fordesalinating sea water.

2. Description of the Related Art

An apparatus of that kind is described for example in WO 02/41979 A1. Inthat apparatus the salt water is introduced under a first pressure intoa pressure compensating device and from there passed under a secondhigher pressure into a membrane module. In the membrane module, it isseparated into desalinated water and concentrated salt water. Thedischarged concentrated salt water which is approximately still at thesecond pressure is continuously introduced into the pressurecompensating device again and is used therein for subjecting the saltwater introduced into the pressure compensating device to approximatelythe second pressure and for introducing the salt water into the membranemodule. More specifically the pressure compensating device describedtherein has two piston/cylinder devices which operate in opposite phaserelationship and the pistons of which are fixedly connected together bya piston rod which is additionally driven.

In desalination installations of that kind which operate on the basis ofthe principle of reverse osmosis, separation into concentrated saltwater and desalinated water is effected at a so-called ‘crossflow’membrane disposed in the membrane module. In the case of such amembrane, the salt water introduced flows along the surface of themembrane while a part thereof passes as desalinated water (drinkingwater) in a direction perpendicularly thereto through the membrane. Themutually crossing flows of water are also referred to as ‘crossflow’. Inthat case the flow on the surface of the membrane also flushes awayunwanted foreign bodies on the surface of the membrane and accordinglytherefore provides for continuously cleaning of the membrane.

In the known configuration of the desalination apparatus having twopiston/cylinder devices, a sufficiently high pressure is admittedlypresent at the moment of switching over the direction of movement of thepistons, to further press water through the membrane and thus producedesalinated water. It has been found however that the crossflowcollapses at the time of switching over the direction of movement. As aresult, at that moment the membrane is no longer sufficiently flushed sothat the situation can involve salt molecules becoming concentrated onthe surface of the membrane, and that can result in a rise in osmoticpressure and thus the operating pressure to the stage of a salt crustbeing formed on the surface of the membrane and operation beingpermanently interrupted.

U.S. Pat. No. 4 187 173 and EP 0 018 128 A1 disclose a method of and anapparatus for desalinating water on the basis of reverse osmosis,wherein a respective pressure compensating container is provided both inthe feed water circuit and also—in U.S. Pat. No. 4 187 173—in theconcentrate circuit. Those pressure compensating containers are in theconfiguration therein of pulsation dampers or differential pressuredampers, in which a piston is displaceable in a cylinder and subdividesthe interior of the cylinder into two chambers. For discharge of feedwater disposed in a chamber, it is provided therein that pressure isapplied to the piston by means of concentrate introduced into the secondchamber, and a spring disposed in that chamber.

FR 2 568 321 and EP 0 055 981 A1 disclose further apparatuses for andmethods of reverse osmosis.

BRIEF SUMMARY OF THE INVENTION

According to principles of the present invention methods and apparatusesfor continuously desalinating water by reverse osmosis which operatewith a described membrane module, provide measures for avoiding thedisruption of the desalination process.

In accordance with the invention that object is attained by a method asset forth in claim 1.

A corresponding apparatus for resolving the problems described is setforth in claim 4. Advantageous configurations of the method according tothe invention and the apparatus according to the invention are recitedin the dependent claims.

In that respect, the invention is based on the realization that theproblems described, in particular an interruption in operation by virtueof contamination and fouling of the membrane surface or indeed damage tothe membrane can be avoided by the flow over the membrane beingcontinuously maintained by suitable means. In accordance with theinvention, provided for that purpose is a reservoir which acts on thesalt water introduced into the membrane module and which, to maintainthe flow over the membrane, additionally introduces water, in particularsalt water, into the membrane module.

In accordance with the invention there is further provided apiston-cylinder device having a piston which subdivides the cylinderinterior into three chambers, wherein the salt water flowing out of thepressure compensating device is present in an inlet chamber, theconcentrated salt water flowing out of the membrane device is present inan outlet chamber and a medium stored in a pressure reservoir, forexample also water or a hydraulic liquid, is present under a highpressure in a pressure chamber. In that respect the desired effect ofmaintaining the flow by the discharge of water from the reservoirpreferably occurs of its own accord. It is however also possible toprovide a suitable control device for controlling the piston/cylinderdevice in order to afford the desired pressure-assisting effect.

Some possible configurations of that piston-cylinder device are recitedin claims 6 and 7.

It is preferably provided that, for example at the switching-over timein the case of the known apparatus with two piston/cylinder devices, apressure drop or flow drop is bridged over in order to maintain thecontinuous flow over the membrane. By way of example suitable sensorscan be provided for measuring a reduction in the flow over the membrane.

In accordance with the invention, there are preferably provided twopiston/cylinder devices which operate in opposite phase relationship, asare known from WO 02/41979 A1. The reservoir then provides that, upon achange in the direction of movement of the pistons, that is to say inparticular at the moment when the pistons are stationary, an assistingpressure is exerted on the salt water. Thus in particular at thatswitching-over time, a possible pressure drop is compensated and theflow is maintained over the membrane.

A further advantageous configuration is provided in claim 3. In thatcase, the pressure required for discharge of the water from thereservoir is produced on the one hand from the pressure of theconcentrated salt water discharged from the membrane module and inaddition from a pressure stored in a pressure reservoir, wherein thepressure which results overall must naturally be greater if necessarythan the pressure of the salt water flowing out of the pressurecompensating device.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention is described in greater detail hereinafter with referenceto the drawing in which:

FIG. 1 shows a block circuit diagram to explain the method according tothe invention, and

FIG. 2 shows an embodiment of an apparatus according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The block circuit diagram in FIG. 1 shows a delivery pump 1 forintroducing salt water 10 into a pressure compensating device 2 under afirst pressure p1. The same salt water 11 which however is now subjectedto a high working pressure p2 is passed from the pressure compensatingdevice 2 to the membrane module 3. There a part of the salt water 11passes through the membrane 6 which is preferably in the form of aso-called crossflow membrane, for example 25% of the salt water 11, itis desalinated in doing so and it is discharged in the form ofdesalinated water 12. The remaining part of the salt water 11, forexample 75%, cannot pass through the membrane 6 but flows along thesurface of the membrane 6 into the connecting conduit 5, by way of whichit is discharged from the membrane module 3 as concentrated salt water13. The concentrated salt water 13 which in that case is still at a highpressure which approximately corresponds to the pressure p2 but issomewhat lower is then passed to the pressure compensating device 2again. There, that high pressure p2 is used in a manner that is still tobe described in detail hereinafter for the purposes of acting withpressure on the salt water introduced into the pressure compensatingdevice 2, and feeding it to the membrane module 3 at the inlet thereof.At the same time that pressure is used in the pressure compensatingdevice to definitively discharge concentrated salt water 14 therein, byway of the discharge conduit 4, and to feed unconcentrated salt water 10to the pressure compensating device 2. All the described procedures takeplace in that case simultaneously and continuously so that there is noneed for a high-pressure pump for subsequently delivering the highworking pressure and desalinated water 12 is continuously available.

As was described hereinbefore, particularly when using a crossflowmembrane 6 it is necessary for the flow of the salt water over thesurface of the membrane to be maintained continuously and under auniformly high pressure as otherwise salt molecules can be deposited atthe surface of the membrane, and such molecules can result in damage tothe membrane or an interruption in operation of the system. By virtue ofvarious circumstances however it can happen that the pressure p2 of thesalt water discharged from the pressure compensating device 2 brieflyfalls so greatly that the flow over the surface of the membrane would bereduced or even interrupted. Desalination would then admittedly stilltake place; it will be noted however that the membrane could be damagedas the concentrated salt water 13 cannot flow away out of the membranemodule 3. In order in such a situation to maintain the pressure p2 andthe flow, there is therefore provided in accordance with the invention areservoir 15 which in such a situation passes additional water into themembrane module 3 and thus ensures that the high working pressure p2remains maintained and the flow over the surface of the membrane is notreduced.

FIG. 2 shows a specific configuration of an apparatus according to theinvention. It has two identical piston/cylinder devices 401, 402 withtwo aligned mutually opposite cylinders which each have a respectiveinlet chamber 201, 202 for receiving the salt water and a respectiveoutlet chamber 101, 102 for receiving the concentrated salt water 13.Arranged within each of the piston/cylinder devices 401, 402 is arespective special piston 301, 302 which subdivides the piston interiorinto the above-mentioned chambers and which in the Figure isdisplaceable in the horizontal direction within the piston/cylinderdevice.

From the delivery pump 1, a respective feed conduit with a passivenon-return valve 7 leads to the inlet chambers 201, 202. The non-returnvalves 7 in that case are of such a configuration that they open andpermit a through flow when the pressure in the feed conduit is greaterthan in the inlet chambers 201, 202. Comparable non-return valves 8which however involve a different through-flow direction are disposed inthe feed conduits from the inlet chambers 201, 202 to the membranemodule 3.

In contrast, actively switchable main valves V3, V6 and V1, V4respectively are arranged in the feed conduits 5 from the membranemodule 3 to the outlet chambers 101, 102 and in the discharge conduits 4from the outlet chambers 101, 102; the feed flow of the concentratedsalt water 13 from the membrane module 3 and the discharge flow of theconcentrated salt water 14 out of the pressure compensating device 2respectively can be controlled by way of the above-mentioned mainvalves.

The pistons 301, 302 are fixedly connected together by means of a pistonrod 30. Pinions 40 which for example can be driven by electric gearmotors and which engage into a tooth arrangement provided on the pistonrod 30 can drive the piston rod 30 and by way thereof the pistons 301,302 in order to compensate for pressure losses.

The pistons are arranged in such a way that they operate in oppositephase relationship. When therefore one piston is disposed in a positionin which the volume of the inlet chamber 202 is at a maximum and thevolume of the outlet chamber 102 is at a minimum, then the other pistonwhich is connected by way of the piston rod 30 is in a position in whichthe volume of the inlet chamber 201 is at a minimum and the volume ofthe outlet chamber 101 is at a maximum (see FIG. 2). In that situationthe inlet chamber 202 is filled with water and the outlet chamber 101 isfilled with concentrated salt water. The valves V1, V3, V4 and V6 whichare illustrated here as switches are controlled in such a way that V3and V4 are now closed while V1 and V6 are opened.

In this connection opening a valve signifies producing a flowcommunication in order to allow a through-flow, for which purpose thevalve is purely mechanically opened. Similarly closing a valve signifiesinterrupting a flow communication in order to prevent a through-flow,for which purpose the valve is purely mechanically closed.

By virtue of the main valve V1 being open, firstly the pressure of theconcentrated salt water in the outlet chamber 101 escapes. By virtue ofopening of the main valve V6 the outlet chamber 102 is subjected to theeffect of pressure (for example about 65 bars) and the concentrated saltwater flows into that chamber. At the same time the salt water disposedin the inlet chamber 202 is pressed to the membrane module 3 by thepiston subjected to pressure.

As the pistons are arranged in such a way that they operate in oppositephase relationship, introduction of the concentrate which is subjectedto pressure (for example 65 bars) into the outlet chamber 102 by thepiston rod 30 causes movement of the other piston 301 which as a resultempties the pressure-less outlet chamber 101. At the same time a reducedpressure is produced in the inlet chamber 201 and sucks in the saltwater and fills that chamber.

When the outlet chamber 102 is filled the main valves are suitablycontrolled and the opposite procedure takes place.

As the membrane module is preferably operated at about 70 bars in orderto provide for a sufficiently high level of fresh water, and at amaximum about 5-10 bars occur as a pressure loss at the membrane, atleast the above-mentioned pressure of about 65 bars of the concentratedsalt water is still available at the concentrate discharge 5 of themembrane module 3.

In order to maintain the flow of the water along the surface of themembrane 6 during the operation of switching over the direction ofmovement of the pistons 301, 302, in particular at the moment when thepistons 301, 302 are stopped, in accordance with the invention there isprovided an additional piston/cylinder device 403, referred tohereinafter as the piston reservoir. It has three chambers, namely afeed water chamber, inlet chamber 203, which is connected to the feedconduit for the salt water 11 which is fed in, a concentrate chamber,outlet chamber 103, connected to the concentrate conduit 5 and apressure chamber 503. In that arrangement the pressure chamber 503 isconnected on the one hand by way of an active valve V7 to the feedconduit 11 and on the other hand directly to a pressure reservoir 20,preferably a bladder reservoir. During operation the valve V7 is alwaysclosed, it only serves to be able to fill up the circuit comprising thepressure chamber 503 and the pressure reservoir 20 again with thepressure fluid, for example a hydraulic fluid, after an interruption inoperation, and to restore the required high pressure in the pressurereservoir 20.

If the effective piston area of the piston 303 in the concentratechamber 103 is about three quarters of the piston area in the feed waterchamber 203 and the piston area in the pressure chamber 503 is about aquarter of that area, the pressure distributions are as follows. Thefeed water chamber 203 is subjected to a pressure of about 70 bars inoperation. That results in up to 280 bars in the circuit comprising thepressure chamber 104 and the pressure reservoir 20. They are however notattained in operation. The operating pressure in that region is betweenabout 200 and 210 bars.

At the time of switching over the direction of movement of the pistons301, 302, a pressure of about 70 bars acts on the piston 303, from thefeed water chamber 203. The pressure in the reservoir 20 is only 160bars. Then, from there, because of the smaller piston area in thepressure chamber 503, the pressure acting is about 160/4, that is to sayabout 40 bars. The pressure in the concentrate circuit, that is to saythe pressure of the concentrated salt water 13 discharged from themembrane module 3, is about 68 bars. That pressure acts on an area whichincludes three quarters of the piston area. Consequently a pressure ofabout 51 bars acts here. Those two pressures act in the same directionand are thus added to give a total of about 91 bars. Only theapproximately 70 bars in the feed water chamber 203 acts against thatresulting pressure. Accordingly there is a sufficiently high pressure topress the piston 203 downwardly in the illustrated position and thus tomaintain the flow over the membrane 6.

Even if it is only a pressure of about 60 bars that is taken as thebasis for the concentrate circuit, that still affords a proportion of 45bars in the concentrate chamber 103. Even if the pressure in thepressure reservoir 20 is only 120 bars, that results in a further 30bars, so that there is still an overall pressure of 75 bars, whichallows the flow over the membrane 6 to be maintained.

The piston reservoir 403 can be controlled in such a way that it is onlyin the case of a pressure drop in the connecting conduit between theinlet chambers 201, 202 and the membrane module 3 or a reduction in theflow over the membrane 6, that an additional pressure is exerted on thatconnecting conduit. For that purpose it is possible to provide forexample suitable sensors which detect such a pressure drop or areduction in flow and which trigger the appropriate pressure controlprocedure. In addition it is also possible to provide valves which aresuitably controlled for that purpose in the concentrate conduit 5between the membrane module 3 and the piston/cylinder device 403, whichare opened if necessary, in order to produce the described movement ofthe piston 303 downwardly by the introduction of a pressure into theconcentrate chamber 103. If in contrast such pressure assistance is notrequired, such a valve can also be closed again so that, because of thehigher pressure in the feed water chamber 203 in relation to thepressure chamber 503, the piston 303 is moved upwardly again and thusremains virtually in the readiness position.

In the case of the piston reservoir 403 according to the inventionhowever such a control can be omitted as it can automatically set thespecified pressure conditions in operation and the desired effect isachieved by separate control. On the one hand then feed water can flowout of the chamber 203 and on the other hand concentrate can flow out ofthe membrane module 3, into the chamber 103, so that the flow over themembrane 6 is maintained.

In addition it is also possible to additionally provide secondary orbypass valves parallel to the described main valves V1, V3, V4, V6 inorder to reduce the loading on the main valves and thus to increase theservice life thereof. In addition it is also possible to provide one ormore quantitative flow limiting devices which are intended to prevent anabrupt pressure compensation effect insofar as they limit the maximumquantitative through-flow and thus contribute to a gradual compensationof pressure and slow changes in pressure, instead of abrupt pressurefluctuations. Elements of that kind and further elements are describedand illustrated in above-mentioned WO 02/41979 A1 to which reference ishereby expressly directed and the description of which is to be deemedto be included herein. The basic mode of operation of such an apparatuswith two piston/cylinder devices is also discussed in detail therein,and reference is also made thereto.

The invention can also be used in relation to apparatuses of a differentconfiguration for the desalination of water by reverse osmosis, whichfor example, instead of the illustrated two piston/cylinder devices,have another number of such devices, for example one or threepiston/cylinder devices. They can in principle also be of a differentconfiguration. The configuration of the reservoir in the form of thepiston/cylinder device with three chambers, as is shown in FIG. 2, isalso not absolutely necessary but in principle can also be of adifferent nature.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A method of continuously desalinating water by reverse osmosis,comprising: introducing salt water under a first pressure by means of adelivery pump into a pressure compensating device having apiston/cylinder device with a salt water chamber and a concentrated saltwater chamber, introducing salt water from the salt water chamber of thepressure compensating device at a second increased pressure into a saltwater chamber of a membrane module and separated therein by means of amembrane into desalinated water and concentrated salt water, dischargingthe concentrated salt water from salt water chamber of the membranemodule at approximately the second pressure, introducing theconcentrated salt water under approximately the second pressure into theconcentrated salt water chamber of the pressure compensating device,wherein the concentrated salt water introduced into the concentratedsalt water chamber of the pressure compensating device acts withapproximately the second pressure on the salt water introduced into thesalt water chamber of the pressure compensating device and acts on thesalt water introduced into the membrane module, and maintaining acontinuous flow of the salt water over a surface of the membrane in themembrane module by means of salt water discharged from a pistonreservoir having a pressure chamber, an outlet chamber, an inlet chamberand a piston, the piston having a front side facing the inlet chamberand a rear side having a first portion facing the outlet chamber and asecond portion facing the inlet portion, each one of the front side ofthe piston and the first portion and the second portion of the rear sideof the piston having a respective surface area, the inlet chamberconnected to the salt water chamber of the pressure compensating deviceand the salt water chamber of the membrane module, the outlet chamberconnected to a concentrated salt water outlet of the membrane module,and the pressure chamber connected to a pressure reservoir, and applyingan assisting pressure from the pressure reservoir to the pressurechamber, wherein the respective surface areas front side of the pistonand the first portion and the second portion of the rear side of thepiston have ratios such that the assisting pressure helps to produce atpredetermined moments in time a respective pressure, which is greaterthan the second pressure of the salt water discharged from the pressurecompensating device, in the inlet chamber.
 2. A method according toclaim 1 characterised in that the pressure compensating device has twopiston/cylinder devices, each of the two piston/cylinder devices havinga respective piston and further comprising operating the twopiston/cylinder devices of the pressure compensating device in oppositephase relationship, and wherein maintaining a continuous flow of thesalt water over a surface of the membrane in the membrane module bymeans of salt water discharged from a pressure booster further includespassing water from the inlet chamber of the pressure booster into themembrane module upon a change in a direction of movement of the pistons.3. A method according to claim 1 further comprising applying theapproximately second pressure of the concentrated salt water dischargedfrom the membrane module to the outlet chamber to help produce therespective pressure in inlet chamber.
 4. A method according to claim 1further comprising: recharging the pressure reservoir via a salt waterconduit connecting the pressure reservoir and the salt water inlet ofthe membrane module.
 5. Apparatus for continuously desalinating water byreverse osmosis, comprising: a pressure compensating device having apiston/cylinder device and defining a salt water chamber for receivingand discharging salt water and a concentrated salt water chamber forreceiving and discharging concentrated salt water, the piston/cylinderdevice receiving salt water at a first pressure and discharging the saltwater from the salt water chamber at a second pressure that is greaterthan the first pressure, a delivery pump for introducing salt waterunder the first pressure into the salt water chamber of the pressurecompensating device, a membrane module having a membrane for separatingintroduced salt water into desalinated water and concentrated saltwater, the membrane module receiving the salt water at the secondpressure from the pressure compensating device via a salt water inletand discharging concentrated salt water at a concentrated salt wateroutlet, and a pressure booster for maintaining a continuous flow of thesalt water introduced into the membrane module over a surface of themembrane by the discharge of salt water from the pressure booster intothe membrane module, the pressure booster having a piston reservoir witha piston, a pressure chamber, and a pressure reservoir, the pistonhaving a front side with a respective surface area and a rear side witha respective surface area, wherein at the front side of the piston, thepressure booster has an inlet chamber connected to the salt waterchamber of the pressure compensating device and the salt water inlet ofthe membrane module and at the rear side of the piston, the pressurebooster has an outlet chamber connected to the concentrated salt wateroutlet of the membrane module and the pressure chamber which isconnected to the pressure reservoir, and that the surface area of thefront side of the piston and the surface area of the rear side of thepiston have a ratio such that at predetermined moments in time arespective pressure is produced in the inlet chamber of the pressurebooster, wherein the respective pressure is greater than the secondpressure of the salt water discharged from the pressure compensatingdevice.
 6. Apparatus according to claim 5 characterised in that thepressure compensating device has two piston/cylinder devices operatingin opposite phase relationship and each having a respective piston andthat the pressure booster passes salt water out of the pressure boosterinto the membrane module upon a change in a direction of movement of thepistons.
 7. Apparatus according to claim 5 characterised in that thepiston is of such a configuration that a pressure obtained in thepressure chamber can act on approximately a quarter of the surface areaof the piston rear side and a pressure obtained in the outlet chambercan act approximately on three quarters of the surface area of thepiston rear side.
 8. Apparatus according to claim 5 characterised inthat the pressure reservoir has a pressure which is at least double thesecond pressure.
 9. Apparatus according to claim 5 wherein the pressurereservoir is in fluid communication with the salt water inlet of themembrane module.
 10. Apparatus according to claim 5 wherein the pressurechamber is in fluid communication with the salt water inlet of themembrane module.
 11. Apparatus according to claim 5 further comprising:a first fluid conduit connected to the pressure reservoir; and a valveconnected to the first fluid conduit, the valve providing fluidiccommunication between the salt water inlet of the membrane module andthe pressure reservoir.
 12. Apparatus according to claim 5 furthercomprising: a first fluid conduit connected to the pressure reservoir;and a valve connected to the first fluid conduit, the valve providingfluidic communication between the salt water inlet of the membranemodule and the pressure reservoir.
 13. A method of continuouslydesalinating water by reverse osmosis, comprising: introducing saltwater under a first pressure by means of a delivery pump into a pressurecompensating device having a piston/cylinder device with a salt waterchamber and a concentrate salt water chamber; introducing salt waterfrom the salt water chamber of the pressure compensating device at asecond increased pressure into a salt water chamber of a membrane moduleand separated therein by means of a membrane into desalinated water andconcentrated salt water; discharging the concentrated salt water fromthe salt water chamber of the membrane module at approximately thesecond pressure; introducing the concentrated salt water underapproximately the second pressure into the concentrated salt waterchamber of the pressure compensating device, wherein the concentratesalt water introduced into the concentrate salt water chamber of thepressure compensating device acts with approximately the second pressureon the salt water introduced into the salt water chamber of the pressurecompensating device and on the salt water introduced into the membranemodule; and maintaining a continuous flow of the salt water over asurface of the membrane in the membrane module by means of salt waterdischarged from a piston reservoir, the piston reservoir having areservoir with a piston, a inlet chamber, a pressure chamber, and apressure reservoir connected to the pressure chamber; the piston havinga front side with a respective surface area and a rear side with arespective surface area, wherein at the front side, the piston is incontact with the inlet chamber of the piston reservoir, and the inletchamber is connected to the salt water chamber of the pressurecompensating device and the salt water chamber of the membrane module;an outlet chamber at the rear side of the piston connected to aconcentrated salt water outlet of the membrane module; said pressurechamber positioned to exert pressure on the piston in the pistonreservoir in operation, and the surface area of the front side to thepiston in the piston reservoir and the surface area of the rear side ofthe piston in the piston reservoir have a ratio such that atpredetermined moments in time a respective pressure is produced in theinlet chamber of said piston reservoir which is greater than the secondpressure of the salt water discharged from the pressure compensatingdevice.
 14. A method according to claim 13 wherein the pressurecompensating device has two piston/cylinder devices, each of the twopiston/cylinder devices having a respective piston and furthercomprising operating the two piston/cylinder devices of the pressurecompensating device in opposite phase relationship, and whereinmaintaining a continuous flow of the salt water over a surface of themembrane in the membrane module by means of salt water discharged from apiston reservoir further includes passing water from the inlet chamberof the piston reservoir into the membrane module upon a change in adirection of movement of the pistons.
 15. A method according to claim 13wherein the respective pressure produced in the inlet chamber of saidpiston reservoir includes combining the approximately second pressure ofthe concentrated salt water discharged from the membrane module with anassisting pressure from the pressure reservoir.
 16. A method accordingto claim 13 comprising: recharging the pressure reservoir via a saltwater conduit connecting the pressure reservoir and the salt water inletof the membrane module.
 17. Apparatus for continuously desalinatingwater by reverse osmosis, comprising: a pressure compensating devicehaving a piston/cylinder device and defining a salt water chamber forreceiving and discharging salt water and a concentrated salt waterchamber for receiving and discharging concentrated salt water, thepiston/cylinder device receiving salt water at a first pressure anddischarging the salt water from the salt water chamber at a secondpressure that is greater than the first pressure, a delivery pump forintroducing salt water under the first pressure into the salt waterchamber of the pressure compensating device, a membrane module having amembrane for separating introduced salt water into desalinated water andconcentrated salt water, the membrane module receiving the salt water atthe second pressure from the pressure compensating device via a saltwater inlet and discharging concentrated salt water at a concentratedsalt water outlet, and a piston reservoir for maintaining a continuousflow of the salt water introduced into the membrane module over asurface of the membrane by the discharge of salt water from the pistonreservoir into the membrane module, the piston reservoir having apiston, a pressure chamber, and a pressure reservoir, the piston havinga front side with a respective surface area and a rear side with arespective surface area, wherein at the front side of the piston, thepiston reservoir has an inlet chamber connected to the salt waterchamber of the pressure compensating device and the salt water inlet ofthe membrane module and at the rear side of the piston, the pistonreservoir has an outlet chamber connected to the concentrated salt wateroutlet of the membrane module; wherein the pressure chamber is connectedto the pressure reservoir, and positioned to exert a pressure into theinlet chamber in said piston reservoir, and the surface area of thefront side of the piston and the surface area of the rear side of thepiston have a ratio such that at predetermined moments in time arespective pressure is produced in the inlet chamber of the pistonreservoir, wherein the respective pressure is greater than the secondpressure of the salt water discharged from the pressure compensatingdevice.
 18. Apparatus according 17 wherein the pressure compensatingdevice has two piston/cylinder devices operating in opposite phaserelationship and each having a respective piston and that the pistonreservoir passes salt water out of the piston reservoir into themembrane module upon a change in a direction of movement of the pistons.19. Apparatus according to claim 17 wherein the piston is of such aconfiguration that a pressure obtained in the pressure chamber can acton approximately a quarter of the surface area of the piston rear sideand a pressure obtained in the outlet chamber can act approximately onthree quarters of the surface area of the piston rear side. 20.Apparatus according to claim 17 wherein the pressure reservoir has apressure which is at least double the second pressure.